pinctrl: mediatek: constify gpio_chip structures

These structures are only used to copy into other structures, so declare
them as const.

The semantic patch that makes this change is as follows:
(http://coccinelle.lip6.fr/)

// <smpl>
@r disable optional_qualifier@
identifier i;
position p;
@@
static struct gpio_chip i@p = { ... };

@ok@
identifier r.i;
expression e;
position p;
@@
e = i@p;

@bad@
position p != {r.p,ok.p};
identifier r.i;
struct gpio_chip e;
@@
e@i@p

@depends on !bad disable optional_qualifier@
identifier r.i;
@@
static
+const
 struct gpio_chip i = { ... };
// </smpl>

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/pinctrl/mediatek/pinctrl-mtk-common.c

@@ -1054,7 +1054,7 @@ static int mtk_gpio_set_debounce(struct gpio_chip *chip, unsigned offset,
 	return 0;
 }
 
-static struct gpio_chip mtk_gpio_chip = {
+static const struct gpio_chip mtk_gpio_chip = {
 	.owner			= THIS_MODULE,
 	.request		= gpiochip_generic_request,
 	.free			= gpiochip_generic_free,